1. Field of the Invention
This invention relates to an image sensing device of a video camera having an automatic gain control circuit and more particularly to an image sensing device which is provided with automatic and manual aperture control devices and which has an improved manual aperture control characteristic.
2. Description of the Prior Art
A video camera includes a lens system which converges light information coming from an object to be photographed; an image sensor which is disposed in a position where an image of the object is formed and converts the light information into an electrical signal; an image circuit which processes a picture signal from the image sensor; a sound circuit; a color modulation output circuit; and others. An automatic light quantity control system is generally arranged within the video camera to automatically adjust a stop aperture so that picture-taking can be promptly started just by directing the camera to an object. Furthermore, in addition to the automatic light control system, a manual aperture control device is provided to permit the photographer to take a picture with some desired effect that is not obtainable by automatic aperture control. Therefore, the photographer can select either automatic aperture control or manual aperture control by operating a switch-over device.
Meanwhile, the automatic gain control system, which is connected to the rear stage of the automatic light quantity control system, controls the level of the picture signal so it does not vary when a quantity of light which is out of a light quantity range controllable by the automatic light quantity control system comes through the lens system. Therefore, when the photographer manually sets a desired aperture value by selecting a manual aperture control mode, the automatic gain control system prevents the picture signal level from becoming a desired level, corresponding to the manually set aperture value.
The following description covers details with reference to FIG. 1 of the accompanying drawings: A lens 1 converges light. A stop device 2 includes an automatic aperture control arrangement, a manual aperture control arrangement and a selection arrangement which permit selection of manual and automatic aperture control modes. An image sensor 3 converts a light signal into an electrical signal. A preamplifier 4 amplifies a picture signal obtained from the image sensor 3. A signal processing circuit 5 performs blanking mixing, gamma correction, etc. on a signal produced from the pre-amplifier 4. A gain control amplifier circuit 6 keeps a signal produced from the signal processing circuit 5 at a predetermined level. An integration circuit 7 converts a signal produced from the gain control amplifier circuit 6 into a DC voltage. The DC voltage thus produced from the integration circuit 7 is used as the control signal for the gain control amplifier circuit 6 for gain control. The circuits 6 and 7 thus form a closed loop which serves as an automatic gain control system. A luminance signal separation circuit 8 separates a luminance signal component from a picture signal produced by the gain control amplifier circuit 6. A chrominance signal separation circuit 9 separates a chroma signal component from the picture signal.
An integration circuit 10 converts a signal produced from the pre-amplifier 4 into a DC voltage. A comparator 11 has an inversion input terminal thereof connected to a reference voltage source 12 and has the DC voltage from the integration circuit 10 applied to its non-inversion input terminal. An aperture control amplifier 13 receives from the comparator 11 a deviation voltage resulting from comparison of the output of the integration circuit 10 and the output of the reference voltage source 12. The amplifier 13 then amplifies an aperture control signal corresponding to the deviation voltage. The output voltage of the amplifier 13 is applied to the stop device 2 for automatic aperture control. A negative feedback loop is thus formed by the stop device 2, the image sensor 3, the pre-amplifier 4, the integration circuit 10, the comparator 11 and the aperture control amplifier 13. The negative feedback loop is called an automatic light quantity control system (or an ALC for short), which controls the aperture of the automatic stop device 2 in such a way as to keep the output of the pre-amplifier 4 from varying.
The prior art device arranged as shown in FIG. 1 operates as follows: The light information on the object which has passed through the lens 1 is adjusted to a suitable light quantity by the stop device 2. The quantity of adjusted light information is then imaged on the image sensor 3. The image sensor 3 then produces a picture signal, which is supplied to the pre-amplifier 4. Upon receipt of the picture signal, the pre-amplifier 4 amplifies the weak picture signal. The output of the pre-amplifier 4 is supplied to the signal processing circuit 5 and also to the integration circuit 10. The signal processing circuit 5 then performs gamma correction on the output of the preamplifier 4 to correct and adjust the overall characteristic of the whole image sensing device to the gamma characteristic of a display device (not shown). Furthermore, the circuit 5 adds blanking pulses to the picture signal from the image sensor 3 and performs a wave form clipping process on the picture signal. The output of the signal processing circuit 5 is supplied to the gain control amplifier circuit 6. A part of the output signal of the gain control amplifier circuit 6 is applied to the integration circuit 7 to be converted into a DC voltage. The DC voltage thus obtained is fed back to the gain control amplifier circuit 6 to keep the output signal of the circuit 6 at a constant level. The picture signal, which is thus controlled at a constant level, is then supplied to the luminance signal component separation circuit 8 and the chrominance signal component separation circuit 9 respectively.
Meanwhile, with the output signal of the preamplifier 4 applied to the integration circuit 10, the circuit 10 converts it into a DC voltage. The DC voltage is applied to the non-inversion input terminal of the comparator 11. The comparator 11 compares the DC voltage with a reference voltage received from the reference voltage source 12. A deviation voltage which results from the comparison is applied to the aperture control amplifier circuit 13. The amplifier 13 then amplifies the deviation voltage and applies it to the stop device 2 for aperture control.
The image sensing device which is arranged in this manner thus controls the stop device 2 by the above-stated automatic light quantity control system in such a way as to keep the output of the pre-amplifier 4 at a constant level. In the event that the light quantity control becomes impossible by the automatic light quantity control system ALC due to a great increase or decrease in the light quantity of the light information on the object, the automatic gain control system which is disposed in the rear stage of the image sensing device performs control in such a way as to keep the output of the gain control amplifier 6 at a constant level. The automatic gain control system thus performs complementary control in place of the automatic light quantity control system ALC of the front stage of the image sensing device.
However, when the image sensing device which is arranged in this manner is to be used in a manual aperture control mode by switching the automatic aperture control mode to the manual aperture control mode, the above automatic gain control system which consists of the gain control amplifier circuit 6 and the integration circuit 7 operates to keep the picture signal at a predetermined level even when the aperture is manually opened or stopped down in an attempt to brighten or darken the picture. In the event of a back-light shot, for example, when the device is set in the manual aperture control mode, a large quantity of light incident on the device operates the automatic gain control system to hinder the desired effect from being attained, as expected from the manual aperture control. This has been a drawback of the prior art device.
It is therefore a general object of this invention to eliminate the above-stated drawback of the prior art device.
It is a more specific object of this invention to provide an image sensing device of the kind having an automatic gain control system and an automatic/manual aperture control selecting arrangement, wherein, when the device is shifted from an automatic aperture control mode to a manual aperture control mode, the brightness level of the picture is freely adjustable by adjusting the manual aperture control.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.